Final answer:
Volcanism in the Cascade Range is due to the subduction of the Juan de Fuca Plate beneath the North American Plate, which causes melting of the mantle and the rise of magma, resulting in the formation of volcanic mountains.
Step-by-step explanation:
The volcanism in the Cascade Range, northwestern United States, is best described as related to plate subduction. This region is part of the Pacific Ring of Fire where the Juan de Fuca Plate is subducting beneath the North American Plate. The process of subduction leads to the partial melting of the mantle above the subducting slab, producing magma that rises to create the volcanic mountain range, including well-known peaks like Mount St. Helens. These tectonic processes have formed the characteristic arc of volcanoes along the Cascades, marking them as distinct from features such as mid-oceanic ridge systems, transform faults, or mantle hotspots like Yellowstone.